An electrochemical cell is the coupling of two electrode materials between an ionic conductor, whereby electrochemical reactions occur at the interface between the ionic conductor, an electrode, and gas. The electrode materials are typically metal or semiconductor and the ionic conductor is typically an electrolyte. Electrodes may also include mixed electronic/ionic materials. Currently, yittria stabilized zirconia (YSZ) is being used as the electrolyte material for certain gas sensors and fuel cells.
Electrochemical cells may operate in open-circuit mode or may be used to drive reactions with the application of current or voltage to the cell. Electrochemical cells are used in many devices, such as gas sensors and fuel cells, and applications, such as electroplating. Electrochemical cells are also used in catalysis for the conversion of reactants into useful byproducts.
A gas sensor is a device that detects the concentration or presence of a single or multiple gas species. A gas sensor may, but need not, include an electrochemical cell. A gas sensor without an electrochemical cell can be considered a non-electrochemical device. A gas sensor may have different transduction mechanisms for detecting gas and may be multifunctional by detecting multiple gas species. For example, potentiometric, amperometric, or impedancemetric transduction mechanisms may be used. One issue with most gas sensors is cross-interference from other species, or poor selectivity.
A fuel cell is a device that directly converts chemical energy into electrical energy for power consumption in applications such as for automobiles and homes. A solid oxide fuel cell (SOFC) is one type of fuel cell that incorporates a solid electrolyte sandwiched between at least two electrodes, one electrode functioning as a cathode and the other electrode functioning as an anode. Fuel cells may also be incorporated into stacks in order to increase power output. In the case where the electrolyte is an oxygen ion conductor, oxygen reacts at the cathode and is transported to the anode through the electrolyte as an ion where the oxygen electrochemically reacts with fuel, such as, for example, H2 or CO, to produce electricity.
In certain devices using electrochemical cells, Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) is incorporated for the enhancement of catalytic reactions through the direct application of voltage or current to the electrodes in an electrochemical cell.
In catalysis, the kinetics of a reaction involves the process of changing rates of intermediate steps and other processes during a reaction. These changes affect the overall reaction rate.
The energetics of a reaction refers to the many different energy barriers to activating the steps of a reaction. As an example, a diffusion barrier is one type of energy barrier. These barriers can be overcome by adding energy to the system. Often thermal energy is used to overcome the barriers.
A reaction pathway is the steps that a reaction follows as it proceeds from starting reactants to final products. The pathway that a reaction follows has to do, in part, with the kinetics and energetics of the system. Adsorption and desorption are processes where gas molecules from the gas phase are trapped (physisorption) or bonded to the surface (chemisorption). These processes often also affect the kinetics and energetics of a reaction.
Surface relaxation involves the motion of entire adlayer(s), while surface reconstruction involves changes in the surface periodicity. Both processes can change the way a reaction proceeds. Surface dynamics may refer to the processes that involve dynamic motion on a surface, such as gas (phase) molecules colliding with a surface or diffusion of a species on a surface.
A catalyst may exist as part of an electrochemical device or atop a “catalyst support” which acts to either provide a certain structure for the catalysts or to disperse the catalyst among different reaction sites.
One major problem in catalysis is diminished conversion due to the presence of “poisons.” The presence of “poisons” can also have negative impacts in gas sensors, fuel cells, and other related devices. Poisons may block adsorption sites or cause phase reconstruction; the latter case may be caused by poisons forming complexes with, for instance, oxygen species from an electrolyte, and possibly followed by desorption of the complex. This may prevent certain mechanisms from occurring that rely on the presence of that oxygen, thereby inhibiting the device or catalyst from performing properly.